1. Field of the Invention
The present invention relates, generally, to thermo-forming machines, and more specifically, to an oven protection system for thermo-forming machines.
2. Description of the Related Art
Thermo-forming machines are used in the manufacture of contoured plastic articles and the like. Machines of this type typically include a source of heat, such as an infrared or convection oven, and a mold. Thus, the ovens may be either gas fired or electric. A plastic, sheet-like work piece is cycled between the oven, where it is heated until it has become plasticized, and the mold where the work piece is formed into a desired shape. The mold may include at least one die and a pre-draw box, pressure plate or match tooling. The mold may thus be vacuum actuated, use air assisted pressure or simple mechanical force, as with match tooling, to form the plastic part. In the case of vacuum actuated molds, the die usually has a perforated surface. A vacuum is applied to this surface through the perforations which draws the heated sheet into contact with the surface of the die thereby imparting the shape of the die to the work piece. The newly formed plastic article is allowed to cool and is removed from the machine. The process is then repeated.
A clamp frame is typically used to secure the sheet-like workpiece in the machine, advance the work piece into the oven until it has become sufficiently plasticized and then to position the heated work piece adjacent the perforated die for molding. Clamp frames typically include a pair of opposed longitudinal frame members as well as a pair of opposed transverse frame members. Each frame member may include one or more clamps used to fix the workpiece relative to the machine.
A screen or other mesh-like material is typically disposed directly above the heating elements of the oven of the thermo-forming machine and beneath the clamp frame to which the workpiece is mounted. Thus, the screen essentially defines the upper portion of the oven. The screen is usually made of some sort of metallic material and because of its proximity to the heating elements, it generally becomes very hot during thermo-forming operations.
In one type of thermo-forming process, a workpiece may be heated to a predetermined temperature in the oven while another is being formed in a mold. In some processes, a final product is being removed from the clamp frame and another workpiece is mounted in the clamp frame while two other workpieces are in process. Thus, once the final product has been removed, a new workpiece is mounted in the clamp frame and is cycled back to the oven. Movement of the workpiece between stations occurs simultaneously. None of the workpieces may be moved until each station is ready to accept it. Alternatively, it is also known in the related art to employ thermo-forming processes having single, double, or any number of stages. Accordingly, those having ordinary skill in the art will appreciate from the description that follows that the present invention is in no way limited to the type of thermo-forming process employed.
During any given thermo-forming process, it is possible that a workpiece may take more time in the mold to form and set up or the operator may take more time than anticipated to remove a final product from the clamp frame and to mount a new workpiece in the clamp frame. In any one of these exemplary situations, the operator may then stop the machine's cycle, for example by pressing an emergency stop button or switch (commonly known as an “E-stop” in the related art). Alternatively, a power outage, mechanical failure, or operator error may interrupt the cycling operation of the thermo-forming machine. In any one of these instances or the like, it is possible that a workpiece may be disposed above the oven for a period that is longer than is necessary to sufficiently plasticize it in preparation for the molding stage of the operational cycle. In this event, the workpiece can essentially melt to the point that it sags downwardly into contact with the hot screen or mesh disposed just above the heating elements of the oven. This causes the workpiece to further melt such that the now melting workpiece drips down into the heating elements of the oven.
When the melting workpiece comes into contact with the screen, heating elements, or other very hot portions of the oven, it can produce smoke, fire, and noxious fumes that may spread throughout the enclosure in which the thermo-forming machine resides. This may cause a shut-down of the factory. If a fire erupts, many of the sensitive components of the thermo-forming machine are destroyed or damaged and must be replaced if the machine is salvageable. The fire can spread to other machines or the factory enclosure itself resulting in catastrophic damage. Even in the absence of a fire, the resulting maintenance that must be performed on the oven to clean the plasticized workpiece material from the heating elements and the associated down time for this effort results in significant costs and a loss of productivity.
In view of these circumstances, it is known in the related art to provide fire suppression systems associated with the thermo-forming machine that typically include fire extinguishers and the like. These devices are triggered in the event of a fire starting in the oven to put it out. Unfortunately, while such systems may save the machine and/or the factory from catastrophic devastation, they do not prevent fires in the first place. In addition, because of the chemicals used in modern fire-extinguishing systems, the thermo-forming machine and oven, in general, and the heating elements specifically, must be thoroughly cleaned before operations may be resumed on that machine. This takes time, labor, and still results in a significant loss of productivity.
Accordingly, there remains a need in the art for a system that allows normal cycling procedures in a thermo-forming machine to occur unimpeded but that prevent a workpiece from coming into contact with the oven of the thermo-forming machine when, for example, it becomes overly plasticized.